In general, the local oxidation of silicon (LOCOS) process is one of the popularly used methods to obtain a device isolation structure by selective oxidation. In a typical LOCOS process, a pad oxidation layer is first grown on a silicon wafer where device elements will be formed, and a pad nitride layer made of an oxidation-preventing mask material is deposited thereon. Then, irradiation and etching are performed using a mask to set a region for forming a device isolation layer. Next, wet etching and dry etching oxidations are performed at a high temperature, so that a thick oxide layer is grown. The grown oxide layer is used as a device isolation layer in the LOCOS process.
However, the LOCOS process can bring about the defective crystallization of silicon wafers due to a “bird's beak” phenomenon by side oxidation and a stress of the pad nitride layer by thermal treatment. This can affect an electric characteristic of the semiconductor device as dimensions of semiconductor devices are more highly integrated.
For this reason, shallow trench isolation (STI) is gradually replacing the conventional LOCOS process for the formation of a device isolation structure.
In an STI process, a trench having a predetermined depth is formed in a silicon wafer. An oxide layer is deposited on the trench and then chemical mechanical polishing (CMP) is performed to polish-etch a redundant portion of the oxide layer, resulting in the formation of a device isolation structure.
However, in the STI process, when the redundant portion of the oxide layer, which fills the trench is polish-etched by the CMP process, the partial edges of the device isolation layer are over-etched, thereby generating a moat. The moat generated at the edge of the device isolation layer causes an electric fringing field when the semiconductor device is operated, thereby generating electrical degradation. Also, a threshold voltage is generated due to a hump, and thus to control a leakage current is difficult. Finally, the semiconductor device becomes unstable in its characteristics and operation.